The pancreas is a complex organ with exocrine and endocrine functions that are a product of different cell lineages. It is generally believed that "precursor" cells reside in the developing pancreatic epithelium and give rise to ductal, acinar and islet cells. However, it is unclear what features distinguish such "precursor" cells and what cues trigger these cells to "transdifferentiate" into the different lineages. In addition, how these cell fate decisions are altered during pathological conditions of the pancreas, e.g. pancreatic ductal adenocarcinoma, also requires elucidation. Two important molecules that serve as "markers" of the lineages include keratin 19 (K19) for ductal epithelial cells and the PDX-1 transcription factor for beta islet cells based upon our studies and that of other groups. We have demonstrated that the K19 promoter is transcriptionaly active in pancreatic ductal epithelial cells (mediated by the GKLF/KLF4 transcription factor) but not in acinar cells. Furthermore, when the K19 promoter is fused to the lacZ reporter gene in transgenic mice, there is evidence of beta-galactosidase expression in pancreatic ductal but not acinar cells. When mice bearing a non-inducible Cre "knock-out" into the K19 locus are bred onto the Rosa26r reporter gene background, beta-galactosidase expression is largely confined to ducts. Thus, we hypothesize that K19 is associated with pancreatic ductal epithelial cell development and cell fate decisions, thereby permitting us to conduct lineage tracings in a rigorous fashion during development and adulthood. We can also dissect the conversion of the normal ductal phenotype into a premalignant/malignant phenotype. The following Specific Aims will be pursued: 1) to determine the transcriptional regulation of the K19 promoter by PDX-1 in different cell lineages and assess whether KLF4 and PDX-1 cooperate in this regulation; 2) To determine differentiation capacities of K19 expressing ductal epithelial cells in mouse pancreas during normal pancreatic development and in adult pancreas. This will be achieved by the generation and characterization of mice in which an inducible Cre recombinase is directed by the K19 promoter and bred into the Rosa26r background, allowing for temporally regulated labeling of K19-positive cells and their molecule progeny; and 3) To generate and characterize K19-mutated Ki-rats transgenic mice in order to determine the molecular basis for ductal metaplasia, a precursor to cancer. In addition, complementary approaches will be utilized to model ductal metaplasia in vitro. In summary, our interrelated studies will provide insights into cell fate decisions during pancreatic development and in adult pancreas and furnish a foundation to define the basis for premalignant stages in pancreatic ductal oncogenesis.